Milling tools of the type generally mentioned above are generally used for recessing slots or gaps in work pieces of metal (or composite material), e.g. with the purpose of providing cogs, spline bars, or the like. In this connection, the surface of the work piece, in which the milling is to be made, may be plane as well as round, e.g. circular or cylindrical as in cogwheels.
In one embodiment, the tool body, the basic body of the tool, has the shape of a circular plate or disc, the plane of which extends perpendicular to the centre axis. Such a tool can only mill out a single slot. With the purpose of providing simultaneous milling of several parallel slots, such tool bodies may be composed to a roll-like construction or long-edge milling cutter having a desired a number of plates situated parallel and close to each other. A disadvantage of the long-edge milling cutter as well as the simple slitting cutter is, however, that the machining becomes comparatively time-consuming and thereby expensive, more precisely as a consequence of the milling having to be carried out in steps in a number of reciprocating relative motions. Thus, in a first step, a fairly shallow slot is produced by the milling cutter being moved rectilinearly in a direction in relation to the workpiece (or vice versa), whereupon the tool is brought back to its initial position for a renewed milling operation, during which the slot is further deepened. These reciprocating motions are repeated until the milled slot has got the desired depth.
In order to make the machining more effective, hob cutters have recently been developed (by those skilled in the art also denominated “hobbing tools”). In such tools, the insert-carrying cams are running along a helical line that is tangent to an imaginary cylinder generated by a straight generatrix parallel to the centre axis. The helicoidal cam is running in a suitable number of turns at a predetermined, uniform pitch. Usually, the screw formation extends in at least four and at most eight turns. In certain embodiments, the tool body is made from one solid piece of material, usually of steel, but in other cases, the same may be composed of a plurality of separate segments that are formed so that the individual cams follow a common helical line. Irrespective of embodiment, such hob cutters are expensive to manufacture. The large investment cost of the tool body is, however, compensated more than enough by the efficiency of the milling cutter.
The tools in question may, independently of the different concrete embodiments of the basic bodies, be equipped with replaceable milling inserts of most varying types. A first variant is denominated full profile insert and includes only one cutting edge, which has a V-shape and can, in one and the same pass, generate the two opposite flanks as well as a gap bottom between two adjacent cogs (or spline bars and the like) to be made. A disadvantage of these milling inserts is, however, just the fact that they have only one usable cutting edge. Therefore, the total machining cost is severely burdened by the costs of the milling inserts.
Furthermore, two types of multi-edged milling inserts occur, viz. milling inserts having either two or four usable cutting edges. The two-edged milling inserts (see, for instance, WO 2011/136275 A1) are formed with one clearance surface and two opposite, identical chip surfaces, which transform into the common clearance surface via two identical cutting edges. When the milling insert is mounted in a seat on one side of the cam of the basic body, one of the cutting edges can remove chips. In order to enable use of the other cutting edge, the milling insert is moved over to a seat in the opposite side of the cam, the previously inactive chip surface being turned forward in the direction of rotation of the tool. In other words, the two-edged milling inserts are non-indexable in one and the same seat. It should also be mentioned that these milling inserts usually include two through holes for two fixing screws.
Four-edged milling inserts, which reduce the insert costs to an absolute minimum, are formed with not only one pair of identical chip surfaces, but also a pair of opposite and identical clearance surfaces, whereby the milling insert obtains four identical cutting edges. In such a way, the two cutting edges adjacent to a first chip surface, after indexing (turning 180°) of the milling insert, can be used in one and the same seat on one side of the cam. To utilize the two other cutting edges, the milling insert is moved over to a seat on the opposite side of the cam, wherein each one of the cutting edges along the second chip surface can work in the same way. Previously known, invertible milling inserts having four cutting edges are formed with a single centrally situated hole for the screw. This may extend either at a right or at an acute angle to the neutral plane of the milling insert.
The invention concerns itself exclusively with milling tools that are equipped with four-edged milling inserts of the type initially mentioned.
When previously known milling tools having four-edged milling inserts are used for the gear milling of, for instance, cogwheels, problems may arise as a consequence of the requirements of dimensional accuracy and surface smoothness of the cog flanks varying. In many cases, the requirements of dimensional accuracy are moderate, and therefore the precision that is obtained only by milling by the use of simple standard inserts is fully satisfactory. In other cases, the requirements are greater, involving that the cog flanks have to be ground or in another way finished after completed milling. In this connection, it is necessary, during the initial milling, to leave a certain (moderate) grinding allowance in the cog flanks. This is carried out by the use of so-called protuberance inserts. These are formed with bulge-like knobs or protuberances in the part of the milling insert where an otherwise plane clearance surface transforms into an end of the milling insert. The difference between a standard insert and a protuberance insert is seen in a comparison between FIGS. 18 and 19 in the appended drawings. In the standard insert according to FIG. 18, the cutting edge is straight up to the transition to an end, more precisely as a consequence of the clearance surface being plane up to said transition. The protuberance insert according to FIG. 19 differs from the standard insert not only in that the same includes a protuberance between the end of the milling insert and an otherwise plane clearance surface, but also in that the same is somewhat longer than the standard insert, more precisely to generate a gap bottom that is deeper situated between the cog flanks.
The fact that previously known standard inserts and protuberance inserts are differently long, in combination with the same including a single central hole to allow indexing, means that they cannot be used in one and the same tool body. Therefore, workshops that should be able to provide roughed as well as finished cog details have to invest in two separate tool bodies, which individually are extraordinarily expensive, in particular when it comes to hob cutters of a complicated shape.
Another disadvantage of previously known milling tools of the kind in question is based on the fact that the only hole included in the milling insert is centrally placed in the same, i.e., halfway between the two ends of the milling insert as well as the two chip surfaces thereof. If the hole is oriented perpendicular to the neutral plane of the milling insert—to be tightenable in a threaded screw hole oriented perpendicular to the axial support surface of the receiving seat—the tightening of the fixing screw is made more difficult in those cases (long-edge milling cutters and hob cutters, respectively) when the tool includes a plurality of cams situated close to each other. The central location of the hole then means that a key grip included in the head of the screw becomes particularly difficult to access. Considering that the tools include a considerable number of milling inserts, the requisite and fairly frequent insert replacements become time-consuming and cumbersome.